This invention is related to proximity switches which detect the proximity of objects in a state of noncontact. In more concrete terms, the invention is related to an electrostatic proximity switch which detects variations of electrostatic capacity.
There are several types of proximity switch which can be classified according to their principles of operation. One of them is the electrostatic capacity type which operates by the principle explained below. The detecting electrode is located at the front of the switch proper. When an object approaches this detecting electrode, the electrostatic capacity between the detecting element and the earth varies. This capacity variation is detected by an electric circuit and is converted into a switching signal.
Compared with with other types of proximity switches, this electrostatic proximity switch has the distinct feature that it is capable of detecting not only metals and non-metallic substances, but practically all substances including glass, wood, water, oils, plastics, etc. The detection performance of the electrostatic proximity switch is greatly influenced by the internal structure of the switch and by the electrostatic environment around the detecting electrode.
Generally, the case of a switch is of synthetic resin which is formed into a cylinder which has one end closed. The detection electrode mentioned above is installed at the inside of the closed end. The outside of the closed end of the case becomes the detecting surface towards which objects approach. To provide the detecting surface with a strong directivity, the rear of the detecting electrode is covered by a screen electrode. The section of the detecting electrode and the screen electrode is called the detecting section.
Moreover, an electric circuit which detects capacity variations and forms switching signal inputs is installed inside the case. Furthermore, the remaining space inside the case is filled with a resin such as epoxy. This resin protects the electric circuit, etc. from vibration and shields the entire case. This is the basic structure of the electrostatic proximity switch. In a conventional electrostatic proximity switch, the above detecting electrode is formed on a printed circuit board. The detecting electrode is thus a copper foil formed into the specified shape on a copper-plate laminated printed circuit board. This printed circuit is installed in the case so that its electrode formation surface is in contact with the inside surface of the case end.
A conventional printed circuit board uses a glass fiber reinforced epoxy board as a substrate. The glass fiber epoxy board has a comparatively high dielectric constant and this dielectric constance is very variable with temperature. In a conventional electrostatic proximity switch, the detecting electrode is closely attached to a substrate of unstable material which has a high dielectric constant. The above are the causes for lowering the detecting sensitivity, resolution and stability of the proximity switch itself.
In short, because the detecting electrode is attached too closely to a substrate having a high dielectric constant and which is unstable, the capacity variation due to an approaching object being tested cannot be large and consequently, high sensivility detection is not possible. Moreover, because the dielectric constant of the substrate varies widely with temperature, the rate of capacity variation caused by an approaching object is also affected greatly by temperature. Thus, high resolution cannot be made and the stability and reliability are lowered.
Also, when ceramic or Teflon is used as the substrate for printing the above detecting electrode, the dielectric constant of the above materials are comparatively stable with temperature variations and the harm of the temperature variation to the detection efficiency is comparatively little. However, even the above substrate materials have a comparatively high dielectric constant and since the detecting electrode is formed close to the substrate, the detection sensitivity and resolution cannot be raised.